The invention relates to the field of televisions having multiple picture displays for asynchronous video signals, and in particular, to such televisions having a wide display format ratio screen. Most televisions today have a format display ratio, horizontal width to vertical height, of 4:3. A wide format display ratio corresponds more closely to the display format ratio of movies, for example 16:9. The invention is applicable to both direct view televisions and projection televisions.
Televisions having a format display ratio of 4:3, often referred to as 4.times.3, are limited in the ways that single and multiple video signal sources can be displayed. Television signal transmissions of commercial broadcasters, except for experimental material, are broadcast with a 4.times.3 format display ratio. Many viewers find the 4.times.3 display format less pleasing than the wider format display ratio associated with the movies. Televisions with a wide format display ratio provide not only a more pleasing display, but are capable of displaying wide display format signal sources in a corresponding wide display format. Movies "look" like movies, not cropped or distorted versions thereof. The video source need not be cropped, either when converted from film to video, for example with a telecine device, or by processors in the television.
Televisions with a wide display format ratio are also suited to a wide variety of displays for both conventional and wide display format signals, as well as combinations thereof in multiple picture displays. However, the use of a wide display ratio screen entails numerous problems. Changing the display format ratios of multiple signal sources, developing consistent timing signals from asynchronous but simultaneously displayed sources, switching between multiple sources to generate multiple picture displays, and providing high resolution pictures from compressed data signals are general categories of such problems. Such problems are solved in a wide screen television according to this invention. A wide screen television according to the various aspects of this invention is capable of providing high resolution, single and multiple picture display, from single and multiple asynchronous sources having similar or different format ratios, and with selectable display format ratios.
Almost all video products currently available to consumers have a format display ratio of 4.times.3, whereas the format display ratio of video production varies widely. If an aspect ratio greater than 4.times.3 is used for video production, an aspect ratio conversion must be performed before display on a consumer television or picture distortion will occur. One method of aspect ratio conversion is known as letterboxing. Letterboxing maintains more (or all) of the horizontal information, at the expense of the number of displayed lines in each field. A video source produced in the 16.times.9 format would contain 181 lines of video each field when converted to a 4.times.3 letterbox format. The extra lines that are not used in each field can be set to a flat field black (or gray) level. Higher aspect ratio sources would contain proportionally fewer lines per field.
A wide screen television, as described herein for example, can have a format display ratio of 16.times.9. This provides an opportunity to display signals in the letterbox format with greater flexibility. Letterbox signals that were originally produced in the 16.times.9 aspect ratio may be zoomed, that is expanded, vertically to fill the screen, with no loss of horizontal information or distortion. An automatic letterbox detector can be based on the assumption that the video signal will have no active video, that is, a substantially constant luminance value not higher than a threshold value. Alternatively, letterbox detection can be accomplished by calculating two gradients for each line in the video field. The first gradient, designated the positive gradient, is formed by subtracting the minimum value of the previous line from the maximum value of the current line. The second gradient, designated the negative gradient, is formed by subtracting the minimum value of the current line from the maximum value of the previous line. Either of the gradients may have positive or negative values depending on scene content, but the negative values of both gradients may be ignored. If either gradient has a positive value which exceeds a programmable threshold, video is considered to be present on either the current line or on the previous line. These register values can be used by a microprocessor to make a determination of whether or not the video source is in the letterbox format. In yet another alternative, not illustrated in the drawings, the automatic letterbox detector may comprise a circuit for decoding a code word or signal carried by a letterbox signal source which identifies the signal as letterbox format.
An automatic letterbox detector can form part of a vertical size control circuit. The vertical size control circuit comprises a letterbox detector and a vertical display control circuit. The vertical blanking and vertical reset pulses can be transmitted as separate signals or as a single three level signal. The automatic letterbox detection circuit can automatically implement vertical zoom or expansion of the 4.times.3 format display ratio signal which includes the 16.times.9 format display ratio letterbox display. When the output signal becomes active, the vertical deflection height is increased by 4/3, which enables the active video portion of the letterbox signal to fill the wide screen without image aspect ratio distortion.
The vertical display control circuit also controls which part of the overscanned raster will be displayed on the screen, a feature referred to as vertical panning. If the vertically overscanned video signal is not in letterbox format, the conventional display format picture can be zoomed, that is expanded, to simulate a wide screen format. In this case, however, the portions of the picture cropped by the 4/3 vertical overscan will contain active video information. It is necessary to vertically crop 1/3 of the picture. Absent further controls, the top 1/6 and bottom 1/6 will always be cropped. However, picture content may dictate that more of the top than the bottom of the picture is better cropped, or vice versa. If all action is at ground level, for example, a viewer might prefer to crop more sky. A vertical panning capacity enables a choice as to which part of the zoomed picture will be shown and which part will be cropped. This is accomplished in the wide screen processor by the generation of a vertical reset pulse that is delayed by a variable amount from the leading edge of a processed vertical synchronizing pulse, referred to as Vsync.. This vertical reset pulse is then used by the chassis to initiate a vertical retrace. Since the video output of the wide screen processor is not delayed, a vertical panning feature may be created by the appropriate manipulation of vertical blanking and vertical reset pulse delay.
A vertical reset generating circuit can comprise a video line counter and comparator. The inputs to comparator are the output of the line counter and a delay setting in terms of video lines, which corresponds to a desired pan value. If it is desired to pan the video up (which can also be thought of as scrolling the video down) by a slight amount (for example, 4-5 lines), the vertical reset pulse is actually delayed from vertical sync by 262.5 lines (the number of lines per field in an interlaced NTSC system) less the pan value. This difference is the delay setting, or delay value. Whenever the output of the counter matches the delay setting, a start reset signal can be generated.
One problem that is encountered when vertical panning is used occurs during special effects mode of VCR playback. In fast forward mode of a VCR, due to the non-standard nature of the linear tape speed, the number of lines in a field decreases by a number which is dependent on recording mode (SP, LP or SLP) and the linear tape speed. A nominal number of lines per field contained in fast forward mode of VCR playback (SP mode) for this discussion is assumed to be 253.5 lines. If the desired panning value is down by 5 lines, the delay setting would be 262.5 lines-5 lines=257.5 lines. However, a new Vsync occurs every 253.5 lines, resetting the line counter, During this mode, a vertical reset will not occur because the counter will be continually reset without ever reaching the desired delay value, causing a collapse of the vertical raster and possible tube damage.